Process and composition for electropolishing stainless iron and steel



Patented Oct. 26, 1954 PROCESS AND COMPOSITION FOR ELEC- TROPOLISHINGSTAINLESS IRON AND STEEL Terry S.Gamble, Shawinigan Falls, Quebec,Canada, assignor to Shawinigan Chemicals Limited, Montreal, Quebec,Canada, a corporation of Canada N Drawing.

8 Claims. 1

This invention relates to the electrolytic polishing of stainless ironand steel. Stainless iron and steel are defined as iron base alloyscontaining up to 30% chromium, with orwithout substantial proportions,up to 40%, of nickel, and optionally, supplemental small proportions ofmanganese, silicon, molybdenum, columbium, titanium, etc, for specialpurposes.

According to this invention, stainless iron or stainless steel iselectrolytically polished by making it the anode in an electrolytecomprising an acid of the group consisting of sulfuric andorthophosphoric acids and mixtures thereof, water, and a material of thegroup consisting of humic acid and alkaline salts of humic acid.Throughout the specification and claims, the alkaline salts of humicacid include the ammonium salt as well as the salts of the alkalimetals, and the term phosphoric acid is used to refer to orthophosphoricacid. Temperature, voltage, current density, polishing time, and the useof enhancing materials, in this invention, vary within the usual limitscovered by these factors conventional electropolishing processes.

Humic acids are the complex acids derived from the decompositionproducts of vegetable matter, e. g. peat, lignite, and humus. In theexamples of this application, the use of humic acid derived from peat isillustrated, but humic acids from other sources are equally suitable.Processes for the preparation of humic acid are described by Thiessenand Engelder, Isolation of Humic Acids, Industrial and EngineeringChemistry, 22: 1131 (1930) and by Esh and Guha- Sircar, J. Indian Chem.Soc., 17:326-31 (1949) Since humic acid is insoluble in water and mostdilute acids, it is usually prepared by precipitation with strongmineral acid from an alkaline aqueous solution. The alkaline salts ofhumic acid can be prepared by evaporation of the water from an alkalineaqueous solution of humic acid. Humic acid precipitates are somewhatcolloidal in nature and diificult to filter, hence the alkaline salts ofhumic acid, which are more easily isolated, are usually used, and whennecessary, used in acid media to form humic acid in situ.

The proportions of ingredients used in the electrolyte of this inventionmay be varied within fairly wide limits. .The current density requiredfor polishing is governed by the composition of the electrolyte, but inall cases should be at least 0.2 amp. per square inch on the anode, andmay be up to 20 or more amps. per square inch. EX- cessively highcurrent densities :are undesirable because of consequent localoverheatingof, elec- Application November 3, 1951, Serial No. 254,799

trolyte, excessive generation of gas, and increased tendency for pittingcaused by local galvanic action. When sulfuric acid is used, theelectrolyte should contain about 35% to of sulfuric acid, about 10% to60% of water, and about 0.2% to 40% of humic acid or alkaline salt ofhumic acid. When both sulfuric acid and phosphoric acid are used in theelectrolyte, any rela tive proportion of the two is satisfactory, theproportion of humic acid or its alkaline salt being in the range between0.2% and 25 the proportion of water being between 60% and 1%, the partof this latter range below 10 being suitable only for those compositionsin which the mineral acids are present in nearly equal proportions, andthe mineral acids making up at least 30% of the electrolyte composition.The higher proportions of humic acid or alkaline humate make theelectrolyte viscous, hence the lower proportions of humic acid or itssalt are preferred. In all the suitable electrolyte compositions, thepreferred proportion of humic acid or its alkaline salt is 2% to 10% byweight of the composition.

The electrolytes may also contain small proportions of additional agentsadded to enhance the polishing effect of the electrolysis, for examplealcohols, which'have been used in the prior art to enhance the lustre ofthe polished metal. Methanol has been used to replace water up to 10% byweight of the electrolyte composition of this invention.

The time required for polishing depends primarily on the condition ofthe original surface. Polishing times as short as three minutes areoften suflicient for a satisfactory polish, and longer times arerequired at lower temperatures, lower current densities, and lower ratesof action of the electrolyte of the material being polished.

When using sulfuric acid in the electrolyte, the preferred temperaturerange is from 30 to C., and the preferred current density on the anodeis from 0.5 to 2.0 amps. per square inch.

The most desirable composition of the essential ingredients of theelectrolyte has been found to be 50% to 60% by weight sulfuric acid, 2%to 4% sodium humate, remainder water. Sodium humate, the sodium salt ofhumic acid, forms a colloidal precipitate of humic acidin the stronglyacidic electrolyte.

When using phosphoric acid in the electrolyte,

3 the preferred temperature range is from 80 to 100 C. and the preferredcurrent density on the anode is from 0.5 to 2.5 amps. per square inch.The most desirable composition of the essential ingredients of theelectrolyte has been found to be about 60% by weight phosphoric acid,about sodium humate, remainder water. Higher proportions of phosphoricacid and sodium humate increase the viscosity of the electrolyte andfavor the formation of froth during electrolysis. Proportions of sodiumhumate within the operating range, i. e. up to 25% by weight of theelectrolyte, are soluble in the phosphoric acid, but the electrolytefunctions as effectively as when humic acid is present as a precipitate,as I it is in the sulfuric acid electrolyte.

Example I A sample of A. I. S. I. type 304 cold rolled stainless steelwas polished at a current density of 0.66 amp. per square inch and 4.5volts, in an electrolyte consisting of 55% H2SO4, 42% water, and 3%sodium humate held at a temperature of about 60 C. The sample waspolished for 5 minutes, at the end of which time it had a brightmirror-like, polished surface.

Example II A sample of type 304 cold rolled stainless steel was polishedat a current density of 1.7 amps. per square inch at 5 volts, in anelectrolyte consisting of 80% phosphoric acid, 18% water, and 2% sodiumhumate held at a temperature of about 80 C. The sample was polished for5 minutes, in which time it attained a bright mirror surface.

Errample III A sample of type CF-SM cast, partially ferritic, stainlesssteel was polished at a current densit of 4 amps. per square inch at '7volts in an electrolyte consisting of 57% water, 18.5% sulfuric acid,18.5% phosphoric acid, and 6% sodium humate, held at a temperaturebetween 80 and 90 C. The sample attained a bright mirror surface withfive minutes electropolishing.

Although the proportion of humic acid or alkaline humate required in theelectrolyte of this invention may be small, its presence is criticalnone-the-less. An electrolyte containing just sulfuric or phosphoricacid and water will produce a polish on a stainless iron or steel anodeover only a narrow range of temperature and current density, outside ofwhich it will merely etch the anode. Hence the action of humic acid oralkaline humate in producing a bright polish over a wide range of acidconcentration, temperature, and current density is remarkable in view ofthe small proportions which are satisfactory. An aditional benefitattained by the use of humic acid or its alkaline salt inelectropolishing is an unexpected and marked increase in the throwingpower of the current, which is the ability of the current to polish inthe recesses of irregularly shaped objects. A further advantage achievedis the ability to use electrolytes containing higher proportions ofWater than are possible without humic acid or alkaline humate, therebyreducing the consumption of relatively expensive acid.

I claim:

1. An electrolyte for use in the electrolytic polishing of stainlessiron and steel articles wherein the article is employed as an anode,said electrolyte consisting essentially of an acid of the groupconsisting of sulfuric and orthophosphoric acids and mixtures thereof,water, and an amount of a material of the group consisting of humic acidand alkaline salts of humic acideffective to produce a bright polishover a wider range of acid concentration, temperature, and currentdensity than in the absence of the member of said group.

2. An electrolyte, as claimed in claim 1, consisting essentially of 35%to 70% sulfuric acid, 10% to 60% water and 0.2% to 40% of an alkalinesalt of humic acid, all percentages being expressed on a weight basis.

3. An electrolyte, as claimed in claim 2, wherein methanol replaces partof the water up to the extent of 10% by weight of the electrolyte.

4. An electrolyte as claimed in claim 1, consisting essentially of 40%to orthophosphoric acid, 10% to 55% water, and 0.2% to 25% of analkaline salt of humic acid, all percentages being expressed on a weightbasis.

5. An electrolyte for use in the electrolytic polishing of stainlessiron and steel articles wherein the article is employed as an anode,said electrolyte consisting essentially of 0.2 to 25% of a material ofthe group consisting of humic acid and alkaline salts of humic acid, 10%to 60% of water, and a mixture of sulfuric and orthophosphoric acid,said mixture comprising at least 30% of said electrolyte, allpercentages being expressed on a weight basis.

6. The art of electropolishing a stainless iron or steel product whichcomprises (1) making the product the anode in an electrolytic cellcontaining an electrolyte consisting essentially of the electrolyteclaimed in claim 2, and (2) while maintaining the temperature of theelectrolyte in the range from 30 to 80 C., (3) applying a voltage to theelectrolytic cell to maintain a current density of at least 0.2 amp. persquare inch on the surface of the anode, whereby the product iselectrolytically polished.

'7. The art of electropolishing a stainless iron or steel product whichcomprises making the product the anode in an electrolytic cellcontaining an electrolyte consisting essentially of the electrolyteclaimed in claim 4, and, while maintaining the temperature of theelectrolyte in the range from 80 to C., applying a voltage to theelectrolytic cell to maintain a current density of at least 0.2 amp. persquare inch on the surface of the anode, whereby the product iselectrolytically polished.

8. A process for electropolishing a stainless iron or steel articlecomprising making the article the anode in an electrolyte consistingessentially of (1) an acid of the group consisting of sulfuric andorthophosphoric acids and mixtures thereof, (2) water, and (3) amaterial of the group consisting of humic acid and alkaline salts ofhumic acid.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,315,695 Faust Apr. 6, 1943 FOREIGN PATENTS Number CountryDate 682,248 Germany Oct. 20, 1939 OTHER REFERENCES Thorpes Dictionaryof Applied Chemistry, 4th edition (1943), vol. 6, page 284.

Metal Progress, vol. 47 (1945), pages 732 and 733.

1. AN ELECTROLYTE FOR USE IN THE ELECTROLYTIC POLISHING OF STAINLESSIRON AND STEEL ARTICLES WHEREIN THE ARTICLE IS EMPLOYED AS AN ANODE,SAID ELECTROLYTE CONSISTING ESSENTIALLY OF AN ACID OF THE GROUPCONSISTING OF SULFURIC AND ORTHOPHOSPHORIC ACIDS AND MIXTURE THEREOF,WATER, AND AN AMOUNT OF A MATERIAL OF THE GROUP CONSISTING OF HUMIC ACIDAND ALKALINE SALTS OF HUMIC ACID EFFECTIVE TO PRODUCE A BRIGHT POLISHOVER A WIDER RANGE OF ACID CONCENTRATION, TEMPERATURE, AND CURRENTDENSITY THAN IN THE ABSENCE OF THE MEMBER OF SAID GROUP.